Prestressed concrete beams with wooden inserts and method of forming the same



Sept. 15, 1970 J. SCHILLINGER 3,528,209

PRESTRESSED CONCRETE BEAMS WITH WOODEN INSERTS AND METHOD OF FORMING THE SAME Filed Oct. 20, 19s? INVENTUR J4 CK SCH/LL/IVGER ATTORNEYS United States Patent O US. Cl. 52-223 9 Claims ABSTRACT OF THE DISCLOSURE The joist comprises a prestressed concrete joist having three spaced superposed tensioned wire ropes extending the length of the beam and a wooden insert embedded in a side face of the beam to provide a nailing surface. The Wooden insert includes a plurality of wooden strips arranged in end-to-end relation and having a plurality of nails partially driven into opposite sides thereof to extend into the surrounding concrete at an upwardly extending angle. A plurality of longitudinally spaced upwardly projecting yoke formations are provided and comprise a pair of finishing nails forming the sides of the yoke and a roofing nail spaced between the finishing nails, the head of which provides the base of the yoke. In forming the beam, the wooden strips are presoaked in water to preclude absorption of water from the poured concrete, the nails driven into the wood at the abovenoted positions, and the strips placed in end-to-end relation in the bottom of a concrete form with the lower wire rope seating in the yoke formation to wedge the strips against the bottom of the form and thereby maintain the same in position during pouring and curing of the concrete.

BACKGROUND OF THE INVENTION This invention relates to a joist and particularly to a prestressed concrete joist having a wooden insert to afford a wooden nailing surface and method of forming the joist.

In the building construction industry, elongated prestressed concrete members are often employed to form the main structural overhead and flood joists. The nature of the concrete material, however, is not such as to readily receive and hold securely fasteners, such as nails or the like, for joining ceiling, cross furring, etc., to the beams. This problem is aggravated particularly in forming ceilings since the entire weight of the ceiling formation depends from the joist. In prior attempts to avoid this problem, brackets have been mounted externally of the joists by means of special purpose concrete fasteners and furring, plasterboard and the like have been fastened to these brackets. This, however, requires additional material, time and labor, all of which add to the total construction cost. Other prior prestressed concrete joists have been formed with a plastic insert cast integrally within the joist, the insert having a bore opening for receiving a screw whereby an external bracket can be attached to the beam to support cables, fixtures, or the like. This latter type of joist, however, is not readily adapted for securing furring, plasterboard and the like to its undersurface as nails cannot be applied to the plastic inserts and the latter are not coextensive with the joist and, accordingly, do not provide a nailing surface coextensive with the joist.

It is desirable that wooden inserts be provided for concrete joists in order to afford an exposed nailing surface but many problems, so far as is known heretofore unsolved, occur in forming such joists, as well as in retaining the inserts in the joists. A particular problem in casting concrete joists of this type resides in the tendency of the wood to absorb water from the poured concrete and expand. The expansion of the wood continues after the concrete is poured, thus stressing and cracking the concrete. Often the wooden insert can be easily removed from the cracked concrete and thus does not provide a firm base into which nails joining the furring, plasterboard and the like to the joists can be embedded.

Another problem relates to maintaining the Wooden insert in the desired location while the concrete is poured and cured. The wooden insert should be located to expose a side portion thereof through a bottom face of the concrete joist. The relative specific gravities of the concrete and the wooden insert are, however, such that the insert tends to rise or float in the concrete. Displacement of the wooden insert from its predetermined position in the concrete form by this floating action often locates the insert totally within the joist Without a surface or any portion thereof exposed for nailing. Prior attempts to fix the wooden insert in the form during the pouring and curing of the concrete, as by gluing the insert, for example, to the bottom of the form, tend to decrease the withdrawal resistance of the insert and often breaks the insert from the joist when breaking the finished joist from the form.

A further problem relates to the ability of the concrete joist to retain the wooden insert under the applied loading of, for example, a depending ceiling formation. A wooden insert having a rectangular cross section and a side face thereof exposed through the bottom face of a joist would ordinarily have low breakout or withdrawal resistance and accordingly could not support ceiling weights beyond a predetermined maximum value. A wooden insert having a dovetailed cross section has been proposed but this has not proven satisfactory in view of the costs involved in cutting inserts to this cross sec tion, as Well as the larger dimension of wood required and hence additional cost thereof to afford a nailing surface comparable to an insert having a rectangular cross section.

SUMMARY OF THE INVENTION The present invention provides a prestressed concrete joist having a wooden insert coextensive in length with the joist, the wooden insert having a face exposed through the undersurface of the joist to provide a nailing surface coextensive in length with the joist. The wooden insert comprises a plurality of wooden strips arranged in endto-end abutting relation to form a continuous exposed nailing surface through the undersurface of the joist. To form the joist, the strips are presoaked in water for a period of approximately twenty-four hours to expand the same prior to locating the strips in the form and pouring the concrete. In this manner, the strips are presaturated with water and will not absorb water from the concrete, thereby precluding further expansion and subsequent cracking of the completed concrete joist.

Prior to placement of the strips along the bottom surface of the form, a plurality of nails, preferably finishing nails, are partially driven into opposite sides of the wooden strips to project laterally and upwardly at an angle to the plane surface thereof. These nails are located in staggered relation along the opposite sides of the strips and act in shear in the cured concrete joint to preclude withdrawal or breakout of the strips from the joist.

In one form hereof wherein single strands of superposed wire ropes are employed, another set of nails are driven into the upper face of the insert to form a plurality of longitudinally spaced yoke formations. Each yoke formation comprises a pair of laterally spaced driven nails, preferably finishing nails, with a partially driven nail, preferably a roofing nail, of shorter length therebetween forming the base of the yoke. After the superposed wire ropes are longitudinally tensioned in horizontal alignment and when the nailing is completed, the water-saturated wooden strips are placed in end-to-end relation along the bottom of the form. .In accordance with the present invention, the height of hte yoke formation from the bottom surface of the form and the vertical spacing between the lowermost wire rope and the bottom surface of the form are such that the rope seats in the yoke formation and wedges the wooden inserts against the bottom surface of the form. In this manner, the wooden inserts are fixed in the form and the tendency theerof to float in the concrete is eliminated.

In another form hereof wherein superposed pairs of wire ropes are employed, the presoaked wooden strips are laid in end-to-end relation along the bottom of the form intermediate the lowermost pair of ropes. A plurality of blocks, preferably formed of concrete and having laterally spaced slots forming seats for the lowermost pair of wire ropes, are wedged between the pretensioned lowermost ropes and the wooden strip at longitudinally spaced intervals along the form, thereby wedging and holding the wooden strips between the blocks and the bottom of the form when the concrete is poured and sets.

In yet another form hereof wherein superposed pairs of wire ropes are employed, brackets, preferably formed of sheet metal, are preformed to provide a pair of laterally spaced recesses along the upper edge of the bracket and are nailed at longitudinally spaced intervals along the upper surfaces of the wooden strips. The strips are then located between the pretensioned wire ropes and the bottom of the form with the ropes seating in the recesses to wedge the strips against the bottom of the form, thereby holding the strips in the bottom of the form when the concrete is poured and sets.

When the joist is finally cured and broken from the form, the lower faces of the wooden inserts lie flush with the undersurface of the joist and coextensive in length therewith. A nailing surface is thus provided for the length of the joist to which furring strips, plasterboard, etc., may be nailed at any point there along. The wooden strips provide a firm nailing surface which cannot be broken from the concrete joist by virtue of the laterally outwardly angled shear nails embedded in the concrete.

Accordingly, it is a primary object of the present invention to provide a prestressed concrete joist having a wooden insert to afford a nailing surface and a method of forming the same.

It is another object of the present invention to pro vide a prestressed concrete joist having an exposed nailing surface on the underside of the joist coextensive in length with the joist and a method of forming the same.

It is still another object of the present invention to provide a prestressed concrete joist having a wooden insert and method of forming the same wherein the expansion of the wooden insert due to water absorption when immersed in concrete is virtually eliminated.

It is a related object of the present invention to provide a prestressed concrete joist having a wooden insert and method of forming the same wherein the wooden insert is readily and easily located in the concrete form and maintained in such predetermined location during pouring and curing of the concrete.

It is a further object of the present invention to provide a prestressed concrete joist having a wooden insert and method of forming the same wherein the wooden insert is positively prevented from breaking out of the concrete joist.

It is still a further object of the present invention to provide a prestressed concrete joist having a wooden insert and method of forming the same which is low in cost.

These and other objects of the present invention will become more apparent upon reference to the following specification, claims, and appended drawings.

l DESCRIPTION OF THE DRAWING FIGURES FIG. 1 is a fragmentary perspective view of a cornpleted joist formed in accordance with the present invention and having portions thereof broken out and in section for ease of illustration;

FIG. 2 is a perspective view of a wooden insert for use in the joist shown in FIG. 1;

FIG. 3 is a fragmentary plan view of a concrete form prior to pouring of the concrete;

FIG. 4 is an enlarged fragmentary cross sectional view taken about on line 4-4 of FIG. 3 with the concrete poured;

FIG. 5 is a fragmentary perspective view of another form of the invention; and

FIG. 6 is a fragmentary perspective view of a third form thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT There is shown, in the form of the invention illustrat d in FIG. 1, a completed concrete joist, generally designated 10, having three superposed spaced wire ropes 12 which have been pretensioned in the usual manner to prestress the cocrete. It will be understood that the three superposed wire ropes are shown for illustrative purposes only, that a greater or lesser number of wires can be superposed and in other configurations, and that any of the conventional wire ropes used for this purpose, such as /2 inch-7 wire 270 K ASTM A-416-59T stress relieved strand, may be employed. A wooden insert, generally indicated at 14, extends along the undersurface of joist 10 and comprises a plurality of elongated strips of wood 16 arranged in end-to-end abutting relation to provide a nailing surface coextensive in length with joist 10.

Each wooden strip 16 has a plurality of nails 18, preferably galvanized finishing nails, partially driven into the opposite sides thereof at spaced intervals therealong. Nails 18 are partially driven at an angle, preferably 45, to the plane surface of strip 16 into the edges thereof to project laterally and upwardly therefrom. Nails 18 are driven in staggered relation along opposite sides of strips 16 and, in the illustrated form, three nails per side per strip 16 are provided. A plurality of yoke formations, generally indicated at 20, are longitudinally spaced along the upper face of strips 16. Each yoke formation 20 comprises a pair of laterally spaced partially driven nails 22, preferably finishing nails, and an intermediate nail 24, preferably a roofing nail, having a head 26 spaced between outer nails 22 in lateral alignment therewith. The relative lengths of outer nails 22 and intermediate nail 24 are such that, when the nails are partially driven to the extent that the pointed ends thereof extend to the lower face of strips 16 as best seen in FIG. 4, the head and upper shank portions of outer nails 22 form the sides of yoke formations 20 while the head 26 of intermediate nail 24 forms the base thereof. The depth of the yoke formation from the heads of the outermost nails 22 to the head 26 of the intermediate nail 24 is preferably at least equal to the diameter of the wire ropes 12 such that yokes 20 form a seat for the lowermost wire rope for purposes as will presently become clear.

As seen in FIGS. 3 and 4, a plurality of concrete forms, generally indiacted at 30, are provided and arranged in side-by-side relation, each form comprising a pair of slightly downwardly converging sidewalls 32 terminating in a bottom portion 34-. Prior to locating the strips 16 in forms 30, it is an important feature of the present invention that the strips are immersed in Water for a period of preferably about twenty-four hours. In this manner the wood absorbs water and expands to the extent that water absorption from the poured concrete is virtually eliminated and the strips 16 have expanded to their maximum dimensions. Strips 16, accordingly, will not expand during the concrete curing period or later and will therefore not stress the concrete and crack the same when the concrete is finally set. Nails 1-8, 20 and 24 may beapplied to strips 16 either before or after the soaking period.

To form the joist, wire ropes 12 are positioned within forms 30 and tensioned in a well known manner and the soaked and nailed strips 16 are placed in the forms 30 in end-to-end relation such that the lower planar faces thereof lie against bottom surface 34. It is an important feature hereof that the height of the yoke formations 20 from the bottom surface 34 and the height of the lowermost wire rope 12 above bottom surface 34 are such that lowermost wire rope 12 seats within yoke formation 30 between outer nails 22 on the head 26 of intermediate nail 24. The tensioned lowermost wire rope 12, in effect, acts as a fixed member and holds strips 16 against bottom surface 34. Concrete is then poured into molds 30 in the conventional manner and the holding action of the lowermost wire rope 12 against strips 16 precludes lateral shifting and floating of the latter within the pourned concrete whereby strips 16 are continuously maintained in engagement against bottom surface 34. It is important that the relative height relationship between the yoke formations 30 and the lowermost wire rope 12 be maintained so that inserts 16 are held tightly between the lowermost wire rope 12 and bottom surface 34 during pouring and setting of the con crete such that when the joist 10 is removed from the form, the lower planar face of insert 16 will be exposed through and lie flush with the bottom surface 34 of joist 10 to provide a continuous nailing surface coextensive in length with joist 10.

As seen in FIG. 4, a plurality of longitudinally spaced vertically extending rods 38 are placed alongside wire ropes 12. A longitudinally extending wire 42 is welded to the upper ends of rods 38 and is spaced above the upper surface of the poured concrete in the molds to provide means for supporting reinforcing rods to be used in adjoining concrete slabs. A plurality of transversely extending rods 44 are longitudially spaced along molds 30 and are inserted through registering openings 46 in sidewalls 32 of each mold. The openings 46 are located at a distance above bottom surface 34 such that rod 44 passes above the uppermost wire rope 12. This height location is significant as wire ropes 12 are under extremely high tension and may break and whiplash out of forms 30 causing injury to nearby workmen. In the event of a break in tensioned wire ropes 12, rods 44 confine the broken wire ropes within forms 30. Also, when rods 44 are withdrawn from the finished beam 10, a plurality of longitudinally spaced transversely extending openings 48 are left in the joist into which support rods for fastening adjacent formwork may be inserted at the construction site.

As an example of a preferred embodiment of the present invention, joist 10 has a cross section 6 inches wide at the top, inches wide at the bottom, and 8 inches in depth. Joist is formed in approximately to foot lengths and the wooden strips 16 are 8 feet long, having a 1 x 2 inch cross section. With these dimensions, at least three wooden strips 16 are utilized in each joist, it being understood that a shorter strip 16 can be provided to complete the combined length of the 8 foot strips to the desired lengthwise dimension of joist 10. Shear nails 18 and outer yoke nails 20 are 8 penny galvanized finishing nails while intermediate nails 24 comprise 1% inch 6- penny galvanized roofing nails. The vertical spacing of wire ropes 12 from the bottom surface 34 is 2, 4, and 6 inches, respectively. Nails '22 and 24 are partially driven into wooden strips 16 such that the pointed ends thereof extend to the lower face of strips 16 and it is significant that, with the nails thus driven, the height of the lowermost wire rope 12 relative to bottom surface 34 is such that the lower periphery thereof is located at the same height as and bears against the head 26 of roofing nail 24. In this manner, the correct relative heights between the yoke formations 20 and the lowermost wire rope 12 are maintained, the workmen applying the roofing nails 24 such that the pointed ends thereof are driven to but extend no further than the lower face of strip 16. Strips 16 may be formed of any suitable wood, pressure treated pine furring being preferred. Rods 44 are inch in diameter and are spaced longitudinally-apart a distance of 20 inches and spaced 6 inches vertically above bottom surface 34. Rods 44 thus are above the uppermost wire rope 12 at spaced points therealong locating openings 48 1% inches below the upper face of joist 10.

Inthe form of the invention illustrated in FIG. 5, wherein superposed pairs of wire ropes 50 are employed, a plurality of generally rectangular concrete blocks or chairs 52 having recesses or grooves 54 formed at the upper corners thereof, are wedged between the lowermost pair of ropes 50 and wooden strips 16 with the ropes seating in recesses 54. The wooden strips 16 are presoaked and laterally extending inclined shear nails 18 are pro vided as in the previous form. The blocks 52 are spaced along strips 16 similarly as yoke formations 20 are spaced in the previous form and each is preferably about 1 inch wide. With the foregoing arrangement, the lowermost pair of ropes 50 precludes lateral shifting of blocks 52 and bears against blocks 52 to wedge the strips 16 between blocks 52 and the bottom of forms 30, whereby floating action of strips 16 in the poured concrete is precluded.

Another form of the invention is illustrated in FIG. 6, wherein generally rectangular sheet metal brackets 56 are employed to maintain strips 16 butting the bottom of forms 30. Each bracket 56 has a lower leg portion 58 bent at a substantially right angle to overlie a strip 16 and a nail 60 is driven through leg portion 58 securing bracket 56 to strip 16. Brackets 56 are secured at longitudinally spaced intervals along strip 16 similarly as yokes 20 are spaced and inclined shear nails 18 are also provided as in the prior forms. Each bracket 56 is cut out at the upper corners thereof to form a pair of lateral recesses or grooves 62. In use, strips 16 are presoaked as before and brackets 56 secured thereto, the strips being then positioned in end-to-end relation with forms 30 with the lowermost pair of wire ropes 50 seating in recesses 62. In this manner, the lowermost pair of ropes 52 bear against bracket 56 to hold the strips 16 wedged between brackets '56 and the bottom of forms 30.

It is a significant feature of this invention that strips 16 are prevented from breaking out of the finished concrete joist 10 under the applied loading of a depending ceiling. By angling the nails 18 laterally upwardly, nails 18 are placed under shear loading in the laterally adjacent concrete when a ceiling formation is nailed to insert 14. While the wooden strips 16 may shrink somewhat in use due to evaporation of the absonbed water, the resistance to withdrawal of strips 16 or breaking out thereof from joist 10 remains high and approximates the loading required to sever the nails under the applied loading. The strips 16 cannot shift laterally of or outwardly from the joist since the nails project laterally from the strips into the concrete. Additionally, localized stress concentration near the undersurface of joist 10 adjacent the lateral nail locations which would tend to crack the concrete between nails 18 and the undersurface of the joist is precluded by angling the nails upwardly into the joist, thereby providing a sufficient quantity of concrete between the nails 18 and the undersurface of the joist to Withstand cracking stresses at these points.

It is thus seen that the objects of the invention are fully accomplished in that there is provided a prestressed concrete joist having a wooden insert providnig a nailing face coextensive in length with the length of the beam. More over, the wooden insert has been treated prior to the formation of the joist such that the insert cannot absorb water from the poured concrete and subsequently expand and crack the concrete, the insert having high breakout or withdrawal resistance due to the unique disposition of the shear nails located along opposite sides thereof. Additionally, the joist is formed such that the wooden insert is prevented from floating or otherwise being displaced from its location exposing a face thereof through the underlying surface of the joist when the concrete is poured and set. The foregoing joist is low in cost, easily formed, and has a Wide range of utility in building construction, such as providing the structural support, providing nailing surfaces for ceiling formations, brackets for electrical cables, conduits, or the like, and suspended ceilings.

The invention may be embodied in other specific forms Without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by United States Letters Patent is:

1. A composite structural member comprising an elongated concrete joist having planar sides, a wooden insert embedded in and extending lengthwise along one side of said joist, said insert having an exposed side portion facing outwardly of said joist providing a nailing surface on said one joist and means for retaining said insert in said joist including a plurality of projections spaced along said insert on opposite sides thereof and extending from said insert laterally into said concrete joist, said concrete joist being prestressed and having a wire rope coextensive therewith, said wir rope being spaced inwardly a predetermined distance from said one side, the side of said insert opposite said exposed side portion thereof having a plurality of yoke formations longitudinally spaced therealong for receiving said wire rope to locate said exposed side portion relative to said one side.

2. A structural member according to claim 1 wherein said yoke formation comprises at least three laterally spaced upstanding nail-like projections, the outermost projections extending into said joist a distance greater than the inner projection to thereby form the opposite side legs of said yoke formations.

3. A structural member according to claim 2 wherein said inner projection is a roofing nail and said outer projections are finishing nails.

4. A structural member according to claim 2 wherein said exposed side portion of said insert is substantially co planar with said one side of said joist and said projections comprise nails partially driven into said insert.

5. A structural member according to claim 2 wherein said side projections extend laterally upwardly at an angle to said one side face of said joist and are spaced along opposite sides of said insert in staggered relation one to the other.

6. A composite structural member comprising an elongated conc'rete joist having planar sides, a wooden insert embedded in and extending lengthwise along one side of said joist, said insert having an exposed side portion facing outwardly of said joist providing a nailing surface on said one joist side and means for retaining said insert in said joist including a plurality of projections spaced along said insert on opposite sides thereof and extending from said insert laterally into said concrete joist, said concrete joist being prestressed and having a pair of wire ropes coextensive therewith, said wire ropes being spaced inwardly a predetermined distance from said one side, a member disposed between said pair of ropes and the opposite side portion of said insert, said member butting said opposite side portion of said insert, said member having a pair of grooves formed therein for receiving said .ropes to locate said exposed side portion relative to said one side.

7. A structural member according to claim 6 wherein said member comprises a concrete block.

8. A structural member according to claim 6 wherein said member comprises a sheet metal bracket.

9. A structural member according to claim 8 wherein said ibracket has a leg portion bent to overlie said opposite side portion of said insert and means extending through said leg portion for securing said bracket to said insert.

References Cited UNITED STATES PATENTS 728,857 5/1903 Buente 52-369 X 899,112 9/1908 Howard 52368 X 1,064,056 6/1913 De Rehinder 523 68 1,559,749 11/1925 Hall 52368 1,649,909 11/ 1927 McKeon 5 237 6 967,274 8/1910 Hand 52376 FOREIGN PATENTS 557,739 1957 Italy.

848,564 1952 Germany.

856,205 1952 Germany. 1,461,536 1966 France.

JOHN E. MURTAGH, Primary Examiner US. Cl. X.R. 52-368 mg? UNI'IED STATES PA'IENT OFFICE CERTIFICATE OF CORRECTION Patent No. 09 Dated September 15, 1970 JACK SCHILLINGER Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

[- Column 1 line 43, "flood" should read floor Column 4 line 25, j "cocrete should read concrete--. Column 5, line 18, "pourned" should read --poured Column 6 line 38, "with" should read within line 66 "providnig" should read providing Column 7 line 24, after "joist" insert side 'mulhlu I... SEALED SEAL;

Edward M Malian of PM! Officer 

